Unlike social wasps, solitary wasps do not form colonies or work together. Therefore, they are far less of a threat than their social relatives.
Several different groups of predatory solitary wasps (excluding other groups such as the parasitic wasps, ichneumons etc.) are covered here. One group, the potter wasps, differ from all the others in that they are related to the social wasps; they are also in the family Vespidae, in their own sub-family, the Eumeninae. The potter wasps generally use mud to build their nests. Some species build nests shaped like old-fashioned clay pots or jars, hence the common name. They are almost always predators of moth caterpillars. They are generally prey-specific in that each species preys on a limited range of moth species.
Many of the solitary wasps are now classified under the subgroup Spheciformes under the superfamily Apoidea, making them close relatives of the bees. In fact, the bees are supposed to have evolved from a subgroup within the family Crabronidae. Previously, all the members of the superfamily Apoidea which are not bees were classified under their own superfamily Sphecoidea. The term Sphecoid wasps can still be applied to the many members of this group, with three families, Ampulicidae, Crabronidae and Sphecidae being widely distributed worldwide and in Asia.
The wasps of the family Sphecidae are variously referred to as digger or thread-waisted wasps, due to their habits of digging burrows in the ground, and their long petiole ("waist") joining thorax and abdomen. Some species have different nesting habits though. Isodontia species nest in existing cavities and use blades of grass or plant fibres to construct individual cells in their nests. Certain species in the subfamily Sceliphrinae are known as mud-dauber wasps, due to the fact that they make nests of mud, similar to the potters. Their nests are usually long, tubular structures; a new cell is constructed next to the old one, thus these nests are often seen in clusters. Most of the ground-dwelling species of the genus Sphex prey on bush crickets, while the mud-dauber wasps (species of Chalybion and Sceliphron) prey on spiders.
The Sphecidae used to be a very varied and diverse group, but taxonomic reviews have separated many species formerly classified under Sphecidae into the large family Crabronidae. Crabronid wasps vary in size from mere millimetres to a most impressive 40mm or more.
Another family formerly classified within the Sphecidae is the Ampulicidae. Members of this family have a distinctive flattened, elongated appearance, and prey on cockroaches. Members of the genus Ampulex are commonly known as cockroach or jewel wasps.Ampulex compressa is one of the best-known and most widely distributed species, a beautiful metallic blue-green species which preys on pests like the American or Australian cockroaches (Periplaneta americana and Periplaneta australasiae).
The Pompillidae are commonly known as the spider-hunting wasps, due to the fact that all memebers of this family prey solely on spiders. These wasps look superficially similar to the Sphecids, but can be distinguished by not having such an obvious "waist", and also, their legs are generally very long and with distinct claws and spikes. Some species are the largest stinging insects known! These are the famous "tarantula hawks" of the Neotropics (the tropical American regions), and they prey on the largest spiders known, the famous (and often unjustly feared) tarantulas (family Theraphosidae). Like the Sphecids, they are very swift runners on the ground
The last group covered here is the family Scoliidae, sometimes known as the ground wasps, a group of primitive wasps which dig burrows in the ground. Unlike the others, the females do not prepare any sort of nest, but simply dig into the ground, sting their prey (almost always beetle larvae) and lay an egg on the paralysed insect. They then leave the way they came, covering the burrow on their way out. The Scoliids are generally slightly stout wasps, varying in size from a mere 6 or 7mm to the giant 50mm Megascolia procer, a magnificent species marked with black and yellow and with wings shimmering blue-green under good light. They can usually be seen in forests, around leaf litter or in grassy fields, where their flight is generally of medium speed, silent and secretive. Sometimes they dig down into the ground and remain hidden for long periods.
As previously stated, with few exceptions, the solitary wasps will never attack of their own accord (except for some species of Pompilids which nest communally and will defend their nests). All can sting if severely threatened or accidentally touched. Their stings range from mild (potter wasps and small Scoliids) to excruciating (some large Pompillids). Also, while suddenly disturbed in the course of their normal activity, the wasps may circle the source of disturbance threateningly. This is not an attack, but merely an attempt to reorientate. To avoid being stung, simply do not touch them. Also, do not go barefoot in grass or in sandy areas where they are sometimes seen.